Generally, because an electronic device is required to have a reduced thickness, a reduced weight, and a reduced size, electronic components are densely arranged and used in the device. For this reason, a cooling capacity (rotation speed) of a cooling fan, the number of cooling fans, an installation position of a cooling fan, and the like are determined so that the temperature of the electronic component does not exceed the upper limit of the operating temperature. As shown in FIG. 7, the temperature inside the electronic device becomes stable at a predetermined temperature that is lower than the upper limit of the operating temperature of the electronic component. In the electronic device, in order to avoid fast temperature rise of the electronic component and the like, as shown in FIG. 5, the rotation speed of the cooling fan is changed in stages according to the temperature inside the device and the like to control the temperature.
FIG. 8 shows in detail the temperature change inside the electronic device in a state in which the temperature inside the electronic device is stable (a part A shown in FIG. 7).
A temperature change measured by a temperature sensor is shown by a solid line. The threshold temperature B is shown by a dashed line B. The cooling fan is set so as to rotate at the maximum speed when the measured temperature exceeds a threshold temperature B in a normal operation state.
When the measured temperature inside the electronic device exceeds the threshold temperature B, the rotation speed of the cooling fan is increased and the temperature is decreased (D1). When the measured temperature is lower than the threshold temperature B, the rotation speed of the cooling fan is decreased and the temperature increases (D2). When the measured temperature exceeds the threshold temperature B again, the rotation speed of the cooling fan is increased and the temperature is decreased (D3). By repeating this operation, the electronic device makes the temperature inside the device during operation stable so that the temperature of the electronic component does not exceed the upper limit of the operating temperature.
In a generally-used temperature control method, a temperature sensor is installed in the device and the rotation speed of the fan is controlled based on the temperature measured continuously or periodically by the temperature sensor. An example of this method is disclosed in patent document 1.
In patent document 1, a device which generates a control voltage based on the temperature measured by the temperature sensor and controls a rotation speed of a cooling fan based on the control voltage and a cooling method are disclosed.
However, because the device disclosed in patent document 1 operates continuously or periodically, a rotation speed control unit operates even when the temperature is stable. Therefore, a problem in which an electric power is unnecessarily consumed and the rotation speed control unit generates heat by its operation occurs.
As one of the countermeasures against this problem, in patent document 2, a method in which a measurement interval is varied based on the temperature measured by the temperature sensor is described.
In the invention disclosed in patent document 2, when the temperature measured by the temperature sensor is within a predetermined temperature range, the measured temperature is compared with the preceding measured temperature. When the temperature increase is small, the measurement interval is made long and when the temperature increase is large, the measurement interval is made short. Further, a projector in which when the measured temperature exceeds the predetermined temperature range, the fan is rotated at the maximum speed is disclosed.